Low cost, robust, and sensitive diagnostic tests are needed for resource limited settings. However, most point of care diagnostic tests currently on the market require a reader, a power source, and an electronic display to output the results, all of which significantly add to the cost of performing the test. Although some colorimetric test strips can be operated without the use of auxiliary electronics, user interpretation is still required and the results may vary from patient to patient. Accordingly, diagnostic devices that are inexpensive, equipment-free, portable, sensitive, with easy-to-read results would be desirable.